The present invention relates to equipment for measuring the exhaust emissions of internal combustion engines, and more particularly to an apparatus for measuring the contaminate contents of exhaust emissions utilizing a subsonic and sonic flow venturi metering system and means for introducing clean dry air to minimize error due to condensation.
Under present day federal regulations the exhaust emissions from motor vehicles must not exceed specified values of certain contaminates. See Section 1201, Chapter XII, Title 45 of the Code of Federal Regulations, as published in the Federal Register, Vol. 36, No. 128, Friday, July 2, 1971, at pages 12652 et seq. See also Kaufman, U.S. Pat. No. 3,699,814.
The presence of such standards had made it imperative that the exhaust emissions from vehicle engines be tested and analyzed to determine the relative amount of impurities therein. Much effort has gone into the development of equipment for use in this field of exhaust sampling, and it is now known to deliver exhaust gases from an internal combustion engine at an accurately measured flow rate through a test apparatus for purposes of determining and analyzing the relative amounts of impurities. The general scheme of such testing is to utilize a device such as a critical flow venturi to maintain a constant mass flow of diluted sample gas through the apparatus; the diluted sample gas is comprised of the vehicle exhaust gas and dilution air. The admission of dilution air cools the sample, reduces the relative moisture content to eliminate condensation in the sample storage units, and thereby makes the sample easier to work with. The diluted sample is then distributed to various sample storage units for subsequent chemical analysis. Because of the varying amount of vehicle exhaust flow rate during the prescribed driving cycles, the amount of dilution air will also change (higher exhaust flow rate requires lower dilution air, and vice versa) due to the constant mass flow device in the apparatus; to make the tests meaningful, the driving cycles are precisely defined.
A system which satisfies these general requirements is described in the above identified portion of the Federal Register. However, the system described in the Federal Register suffers a number of difficulties and disadvantages, which are discussed in U.S. Pat. No. 3,699,814, to Kaufman, entitled "Gas Sampler," issued Oct. 24, 1972, and now assigned to the assignee of the present invention. The Kaufman patent, the disclosure of which is incorporated herein by reference, taught a much improved gaseous exhaust emissions sampler which replaced the troublesome constant displacement pump of prior systems with a critical flow venturi for metering the diluted exhaust emissions at a constant volume flow.
One problem with lowering the sample temperature is that the cooler sample cannot carry as much water vapor as the heated sample did, with the result that some of the water vapor condenses in the sampling apparatus. This condensation directly affects the volumetric flow through the sampling apparatus with a corresponding loss of accuracy. The conventional technique for addressing this problem is to use heated sample lines to prevent condensation. Heated sample lines are difficult to maintain and add greatly to the complexity of the sampling system.
In addition to the moisture problem and for real-time (modal) testing, there has been a problem with maintaining system precision and accuracy in the dilution air admission system. As stated above, the admission of dilution air must be done with accuracy; and the act of performing the exhaust emission test on the engine must not appreciably alter the engine's operation, otherwise the test results would not be accurate. For example, the exhaust testing apparatus should not place undue back pressure or suction on the engine exhaust system, otherwise the engine would not be operating as it would under normal operating conditions. Conventional exhaust analyzers have had problems in this regard, since conventional flow meters used in such analyzers require high pressures in order to operate in the most accurate range. The need to use conventional flow meters in a comparatively high pressure environment has presented problems in interfacing the high pressure environment of the meter with the engine exhaust system.
Some prior art techniques also utilize tracer methods (carbon dioxide or other nonreactive gases) in which two independent analyzers are used. Such techniques are not considered to be generally satisfactory. In addition, prior art techniques are based on certain assumptions which may not be true in practice, namely that the analyzers introduce no calibration errors and that there are no leaks in the system. There has heretofore been no effective way of calibrating to ensure that these errors and leaks are not inherent in the test system.
Accordingly, the present invention provides an apparatus and method for calibrating an apparatus for measuring the exhaust mass flow of an internal combustion engine having an exhaust conduit. The apparatus includes a test conduit having an exhaust inlet coupled with the exhaust conduit. A subsonic venturi is coupled to the test conduit for enabling passage of fluid. Transducers are coupled with the subsonic venturi for providing signals. A sonic venturi is coupled to the test conduit for enabling passage of fluid. Transducers are coupled with the sonic venturi for producing signals. A computer is coupled with the transducers for interpreting the signals.
The method comprises capping the exhaust inlet of the test conduit; causing a quantity of fluid to flow through the subsonic venturi, through the test conduit, and through the sonic venturi; measuring the flow through the sonic venturi to produce a first signal; measuring the flow through the subsonic venturi to produce a second signal; comparing the first signal and second signal to determine an error; adjusting either of the first or second signals to null the error and to thereby calibrate the apparatus.
Further, the present invention provides an apparatus and method for extracting a proportional sample of exhaust gas from an internal combustion engine. An extraction member is coupled with the engine for providing a confinement path for the sample. A mechanism is coupled with the extraction member for providing clean dry air into the extraction member. A measuring mechanism is coupled with the first mechanism for providing clean dry air into the extraction member. The measuring mechanism produces a first signal. A second measuring mechanism produces a second signal. A computer is coupled with both the measuring mechanisms for interpreting the signals produced. The method comprises extracting a sample of exhaust gas from the internal combustion engine; passing the sample into the extraction member; mixing the sample with a quantity of clean dry air; measuring the quantity of clean dry air with the exhaust sample to produce a first signal; measuring the quantity of the mixed clean dry air and exhaust sample to produce a second signal; determining a ratio of the clean dry air to the exhaust sample; comparing the ratio of the first and second signals with a predetermined ratio to determine an error; and adjusting either the first or second signals to null the error and to thereby produce a proportional sample of exhaust gas.
An advantage of this system is that it provides a means of measuring diluted raw samples on a real time basis using analyzers on same ranges as used for bag analysis. The advantage is a cost savings of several analyzers (or possibly a complete/separate system) for raw only samples.
It is possible to eliminate the need for bag sampling (sample storage units) entirely by integrating the real time gas analyzer readings over the EPA required driving cycles, again resulting in cost savings of the required bag sampling system to the customer. Correlation to the conventional bag sampling system is required, however, if test results are to be used for testing to present EPA requirements.
For a more detailed understanding of the invention, its objects and advantages, references, may be had to the following description and to the accompanying drawings.